Given a list of coordinates {c1,c2, ...., cn} the area of the
polygon defined (returning to c1 at the end) is the sum of the
signed areas of the triangles {c1,c2,c3},{c1,c3,c4},
...{c1,cn-1,cn}. So a formula for it is
Area[cds_] :=
Plus@@Det/@Partition[(# - First[cds])&/@Rest[cds],2,1]/2
Steve's colleague, who needs the areas of fields, can get the list
of coordinate of points along the field boundary in an
anti-clockwise direction in the that way Paul suggests. The areas of
ponds can be be found separately and deleted, or he could make one
list by jumping onto their edges and going round them them
clockwise. And, for islands in those ponds, ..
Allan Hayes
hay at haystack.demon.co.uk